The airways of asthmatic humans are hyperreactive to bronchoconstricting agonists, and it is likely that nerves, mast cells, eosinophils, airway epithelia and airway smooth muscle cells all interact by means of a variety of mediators to increase bronchomotor tone. In addition to these important cell-cell interactions there is also evidence for a postjunctional component of airway hyperreactivity in which mediators interact at the level of airway smooth muscle cells. Preliminary studies demonstrate synergistic effects of low concentrations of histamine and serotonin on the muscarinic response of tracheal and bronchial smooth muscle of the dog. This suggests part of the mechanism of airway hyperreactivity involves postjunctional events of excitation-contraction coupling. To establish the significance and mechanisms of this phenomenon six spasmogenic agents will be tested for synergistic effects: methacholine, histamine, serotonin, prostaglandin F2alpha, and leukotrienes D4 and E4. The aims of the project are to: 1. Define the mechanical effects of synergistic combinations of mediators of bronchospasm. 2. Measure changes in second messengers formed in response to combinations of mediators. 3. Determine whether potentiation of Ca2+-induced contraction by mediators occurs in permeabilized tracheal smooth muscle. 4. Determine the effects of combinations of mediators on contractile protein phosphorylation. 5. Establish the significance and mechanism(s) of potentiation in third and fourth generations of bronchiolar smooth muscle. The results will define the contribution of a postjunctional component to airway hyperreactivity produced by several important mediators of bronchoconstriction. The results will also test the notion that potentiation can occur at several steps in excitation-contraction coupling and it might vary as a function of airway generation. Improved understanding of the mechanism of airway hyperreactivity may contribute to development of improved therapeutic strategies for treatment of asthma.